Objectives: To determine the influence of peripheral input upon the differentiation, and the development of functional organization and related morphology in mechanosensory unclear centers of mammalian brain. Methods: (1) Measure number of cells, (2) trace development of fiber connections, (3) electrophysiologically analyze pattern of projection of receptive fields. In: (1) dorsal root ganglia, (2) cuneate-gracile nuclear complex, (3) ventrobasal thalamus. In Marsupial Opossums: (1) in a normal ontogenetic series, birth to adult, and (2) who have had receptive fields removed by ablation, or detached by serving fiber tracts, before and during the period of nuclear differentiation (2 to 50 days after birth).